1. Field of the Invention
The present invention relates to a braking system; and more particularly, to a braking system having a logarithmic characteristic.
2. Description of Related Art
Known braking systems, used for multiple-disk brakes, include a shaft on which a brake piston is disposed. The brake piston is biased against a friction brake via a compression spring with a linear characteristic. The friction brake of such known systems includes brake disks fixed to the shaft, and brake lamellae located between the brake disks. The brake lamellae are secured in a longitudinally displaceable, but co-rotational fashion to a housing of the brake system. The compression spring biases the brake piston such that the brake piston pushes the brake lamellae and brake disks together; thus, bringing about the braking affect.
To release the brake, such conventional braking systems counteract the biasing effect of the compression spring by hydraulically actuating the brake piston using a liquid or gaseous medium. The more pressure exerted by the hydraulic system, the less the forward movement of the piston in the direction of the brake lamellae, and the less the braking effect. When the brake system is pressureless, the compression spring compresses the brake piston completely onto the friction brake bringing about the greatest braking effect (i.e., the greatest braking torque).
Typically, an annular chamber is formed between the piston and the brake system's housing. Specifically, the piston includes a small diameter portion and a large diameter portion. The small diameter portion extends towards the friction brake, while the large diameter portion extends away from the friction brake. The brake system housing is similarly constructed in that the inner surface thereof has a first portion extending away from the brake system with a diameter substantially equal to the large diameter of the brake piston, and a second portion extending towards the friction brake with a diameter substantially equal to the small diameter of the brake piston. The first portion of the housing has a predetermined length such that the brake piston can be compressed completely onto the friction brake.
The annular chamber is formed between the brake system housing and the brake piston where the brake piston changes from the small diameter portion to the large diameter portion and the brake system housing changes from the large diameter portion to the small diameter portion. When a pressure medium is supplied to this annular chamber, seals disposed on either side of the chamber prevent the pressure medium from leaking, and the pressure acts upon the radial surface of a shoulder created by the piston changing from a small diameter to a large diameter. The pressure acting upon this shoulder causes the piston to move in a longitudinal direction (i.e., an axial direction of the shaft) away from the friction brake. When the force applied by the pressure medium is greater than the force of the compression spring, the brake is completely disengaged.
Such conventional brake systems have a number of disadvantages. Because the compression spring has a linear characteristic, a linear relationship exists between the pressure of the applied pressure medium and the braking torque. Consequently, a relatively large change in the braking effect occurs due to a small change in the pressure of the pressure medium when the pressure of the pressure medium is high. Unfortunately, when the braking effect is low, it is more desirable to be able to gradually increase or decrease the braking effect. Conventional brake systems, however, cannot achieve gradual increases or decreases in the braking effect when the braking effect is low (i.e., when the pressure of the pressure medium is high).
This problem is aggravated by the fact that the seals for maintaining the pressure medium within the annular chamber cause a hysteresis in the movement of the brake piston (i.e., the seals hinder the movement of the brake piston due to friction between the seals and the walls of the piston). This adversely affects the capacity of the brake system to gradually change the braking effect in the low braking torque range. If the friction force of the seal is greater than the force used to apply the braking torques, then small braking torques can no longer be regulated.
Such conventional brake systems combine the cooling and pressure hydraulics into one system. The cooling of the brake lamellae with hydraulic oil, however, does not lead to the intended ideal stick-slip-free braking behavior. This ideal braking behavior does not occur even if, for instance, ATF oils are used in the entire hydraulic system. Even so, these oils are too expensive for a combined cooling and hydraulic system due to the large volume of such a system.
A further problem of known brake systems resides in the difficulties of dismounting the brake linings, and in that the brake lamellae are heated by friction during idling where no braking effect occurs. In other words, even when the brake piston is fully disengaged, the brake lamellae may rub against the brake disks. To solve this problem, the use of spring washers or sine lamellae is necessary. Moreover, as a result of fluid friction, a high torque build up occurs because of the idling friction and torques which vary greatly with the number of revolutions of the shaft.